The Myserious Murasame Castle has very few things that are canon, considering the only installment only provided quick exposition as to why the events of the game are happening like any other NES/Famicom game.
We know that Takamaru is skilled with the sword and throwing weapons, along with fireballs, invincibility, and lightning, who character's traits are being young and his personality only suggesting implications of being determined.
We know the events of The Myserious Murasame Castle take place in ancient Japan, during the time Shogun Togukawa Ietsuna in Yotsuya Tokyo.
We know about how the Murasame statue used to be treasured by the local folk, before being possessed by a mysterious evil force.
We know about the four neighboring castles, where the daimyos were corrupted by the evil force.
So on and so forth.
Meanwhile, something like Murasame Mode in Samurai Warriors 3 is at the moment ambiguous in terms of canon. Obviously Murasame Mode is supposed to be treated like a pseudo-sequel, so it acts like one with more fleshed out characters and story and stuff, but at the same time it can easily be retconned.